How did Lake Nyos suddenly kill 1,700 people?

­Lake Nyos had long been quiet before it happened. Farmers and migratory herders in the West African country of Cameroon knew the lake as large, still and blue.

But on the evening of Aug. 21, 1986, farmers living near the lake heard rumbling. At the same time, a frothy spray shot hundreds of feet out of the lake, and a white cloud collected over the water. From the gro­und, the cloud grew to 328 feet (100 meters) tall and flowed across the land. When farmers near the lake left their houses to investigate the noise, they lost consciousness.

The heavy cloud sunk into a valley, which channeled it into settlements. People in the affected areas collapsed in their tracks -- at home, on roads or in the field -- losing consciousness or dying in a few breaths. In Nyos an­d Kam, the first villages hit by the cloud, everyone but four inhabitants on high ground died.

The valley split, and the cloud followed, killing people up to 15.5 miles (25 kilometers) away from the lake. Over the next two days, people from surrounding areas entered the valley to find the bodies of humans and cows lying on the ground.

By Aug. 23, the cloud had mostly blown away, and the silence had lifted. After being unconscious for up to 36 hours, some people revived to find, horrifically, that their family members, neighbors and livestock were dead.

The lake had changed, too. It was now shallower; plants and leaves floated in it; and its formerly picturesque blue hue had darkened into rust. What was the deadly force at Nyos? Find out next.

More than a week after its eruption, Lake Nyos has lost its blue hue and turned brown. You can also see vegetation damage caused by the water surge that followed the emergence of the toxic gas cloud.

Investigating the Nyos Disaster

­Scientists soon learned that the cloud contained carbon diox­ide (CO2). That finding explained the cloud's heaviness, since CO2 is denser than air. The cloud was actually CO2 mixed with air. The CO2 killed directly by shutting off people's consciousness and breathing. When the CO2 concentration was 15 percent or less, people lost consciousness and later revived. Individuals who inhaled more than 15 percent CO2 stopped breathing in minutes and died.

As for why the lake ejected CO2 -- up to a third of a cubic mile of it -- scientists disagreed. One camp believed a volcanic eruption released CO2 and blew up the lake. Another camp thought CO2 was gradually leaking into and being stored in the lake. When the lake exploded, it released the gas in a gigantic, deadly burp.

"While the two camps of scientists were fighting, they agreed that CO2 killed the people, and the people would be safer on higher ground," says William Evans, a geochemist with the U.S. Geological Survey who investigated the disaster. The Cameroonian government acted accordingly.

Evans' team installed CO2 monitors on the lakeshore. The monitors were hooked to sirens that would sound if too much CO2 entered the air. People knew the alarms meant they should go to high ground.

As years passed, scientists resolved the debate about the origin of the CO2. After measuring gas at the bottom of Lake Nyos, they found a CO2-rich layer, where levels of the gas were rising over time, suggesting gradual leakage into the bottom of the lake.

Scientists looked for markers of a volcanic eruption, such as sulfur and chloride in the lake. They also installed seismometers around the body of water to record tiny earthquakes that would follow a volcanic eruption. "It was the quietest area that the British Geological Survey had ever monitored," says Evans. The volcano hypothesis died. CO2 was bubbling into the lake from below.

Scientists reasoned that CO2 had been trapped in the bottom of Lake Nyos for a long time, held down by 682 feet (208 meters) of water. On the day of the eruption, however, something external triggered the release of gas. Most likely, it was a rockslide from one of the lake's walls. When the falling rocks sunk to the bottom of the lake, they pushed up some gas. Then most of the gas bubbled up.

­If this sounds like a freak occurrence to you, read on to learn about the lake that exploded in an eerily similar fashion just two years before.

Candidate for another exploding lake? Lake Kivu, seen here at dusk on Oct. 3, 2006, in Goma, Democratic Republic of Congo, has CO2 leaking in from magma below and is very deep.

Lake Monoun and Other Exploding Lakes

­Almost two years earlier, on the evening of Aug. 15, 1984, Cameroonians about 62 miles (100 kilometers) southeast of Nyos had heard similar rumblings near a lake. The site of this prior explosion, however, was the smaller Lake Monoun. Around 11:30 p.m., CO2 shot out of the lake and sunk into a valley, near a road. As people from the nearby village of Njindoun walked down the road on their way to work before dawn, they entered the cloud, collapsed and died.

By 10:30 a.m. or so, wind had swept the cloud away. A doctor and police officer arrived on the scene to find most of the 37 dead on a short stretch of road, including a man slumped over his motorcycle [source: Sigurdsson].

The Cameroonian government suspected the explosion was an act of terrorism or the result of someone dumping chemicals into the lake. More traditional villagers in Njindoun believed legends that evil spirits periodically left the lake and killed neighboring people. "Probably these legends came about because of gas bursts in the past," says Evans.

Another lake in Africa may be building toward a burst. Lake Kivu, situated between Rwanda and the Congo in the African Rift Valley, is a legitimate worry. It's more than twice as deep as Nyos and can store more gas. Bacteria in the lake are chugging out methane, and CO2 is leaking in from magma below. Sediment layers suggest the lake may have erupted 7,000 to 8,000 years ago, says Varekamp. Because 2 million people live near Kivu, the lake's gas pressure is being monitored. "If that ever were to go, that would be a natural disaster on a scale we haven't seen, except for the tsunamis in 2004," says Varekamp.

There's also Lake Quilotoa in Ecuador, which is rich in CO2, deep and in a tropical climate. "Some scientists consider it a potential analog of Nyos," says Varekamp.

­You may be wondering whether any lake can explode. Could it happen to the pond in your backyard? Let's return to our historic lakes to find out.

Recipe for a Killer Lake

­Exploding lakes are rare, and the backstories of Lakes Nyos and Monoun explain why. In Cameroon, there are weak spots in the Earth's crust at which magma, or liquid rock, rises from the Earth's mantle. The magma shoots up quickly and vertically, cutting a tube toward the surface. If it reaches the surface, the magma may spurt out and rain a big pile of rock, depositing a cinder cone volcano.

If the magma hits wet rock as it rises, it explodes, blasting a crater in the ground. More than 18,000 years ago, such a blast formed the crater at Lake Monoun [source: Sigurdsson]. A si­milar blast happened a few hundred years ago to form Nyos [source: ­Kling]. Water filled the craters, and they became lakes.

At the bottom of each lake, the old tube where magma first rose to the surface remains. If you follow the tube some 3 to 6 miles (5 to 10 kilometers) down, you'll hit magma. The pressure down there forces out one of the most abundant gases in liquid rock: CO2. The gas rises up the tube into the lake. Researchers have identified more than 100 places in Cameroon where CO2 leaks in large, but not dangerous quantities out of the ground, says Evans.

Several factors -- not just CO2 -- have to align to create an exploding lake. First, the lake must be deep. When little water holds down the gaseous bottom water, the lake needs only a small disturbance -- wind -- to release the gas. In deep lakes, the overlaying water acts as a cork in a champagne bottle. Every 10 meters (33 feet) of water adds 1 atmosphere of pressure, so in a 100-meter (328-foot) lake, 10 atmospheres of pressure hold down the gas at the bottom, says Evans. Wind can't stir it up.

Second, the climate must be stable all year, which is why exploding lakes cluster in the tropics. Lake Superior in the United States, for instance, charges with gas from decaying matter until the season changes. Every fall, the lake's surface cools and gets denser, then sinks to the bottom. The gaseous bottom water rises. The lake turns over, or exhales -- most lakes do at least once a year, says Varekamp. In places where it's warm or cold year-round, lake layers hold their temperature and position. Third, it takes a trigger like a landslide, earthquake or too much gas to unsettle the gas layer.

Cameroon has all the right ingredients for exploding lakes: magma releasing CO2­ into deep lakes, a tropical climate and a trigger.

Degassing Lakes with Big Straws

­After Lake Nyos burst, an international team began discussing ways to degas bot­h lakes and avert future disasters. They talked about bombing the lakes to blow out the gas. But scientists worried a bomb would also blow out one of Lake Nyos's walls, causing an enormous flood. "That would be a disaster in its own right," says Evans. As early as November 1986, French scientists proposed a pipe.

"The pipe idea won out because it's simple, and there's not much risk associated with it," says Evans. "You could eliminate the gas in a controlled fashion."

Pipes were slow to be installed. Money and roads into Nyos weren't plentiful. "When we left Cameroon in 1986, we were certain that we had done good science, recommended how to fix the problem and said aid groups will come in next week and start piping the gas out. It was a wake-up call for all of us for how long this type of stuff takes," says Evans.

The first pipe went into Lake Nyos in 2001. A French engineering team sunk a 6-inch (15-centimeter) plastic tube 666 feet (203 meters) into the lake until it reached the gas layer [source: Halbwachs]. Again, froth shot out like champagne from an uncorked bottle, but this time it wasn't a deadly surprise.

Today, Nyos is degassed to about 80 percent of the level after the 1986 explosion, says Evans. "The lake is safer today than it was in 2000, but it is still hazardous." A big enough input of energy, like a large earthquake or a landslide, could cause the lake to erupt, he says.

Another problem is Nyos' weak wall. "That natural dam could rupture any day," says Evans. "If the dam were to suddenly fail, the upper 40 meters [131 feet] of the lake would empty in a huge flood, and that would release the pressure on the gas remaining in the deep water. You could have a combination of a flood and a gas release." Evans says the gas should be piped out as soon as possible, and then the wall should be fixed. Two more pipes are planned, with the first possibly going in in spring 2009.

Lake Monoun has three pipes: one installed in 2003 and two in 2006. "Another eruption is probably not possible there, given that the lake is almost completely degassed," says Evans. "Monoun now would be a very nice place to live."

­So the next time ­you smell the sulfurous gas as your local lake turns over, think of it as a lake exhalation -- and be thankful that it's not a burp.